In recent years fluidized beds have found many diverse uses in power generation systems and chemical processes. They have served as chemical reactors, particularly for finely divided materials; as incinerators for liquid, solid or gaseous substances; as pressurized or atmospheric, coal-, lignite-, petroleum-, peat-, wood- and/or paper-fired boiler or combustor units for power generation; and, as sites for various process treatments such as drying, baking, coating, oxidizing, etc.
Typically, fluidized beds which are in use today are static beds established when air or other fluidizing gas is introduced into a plenum chamber under pressure and forced upwardly through a diffusing medium (e.g., membrane, grate) to a superimposed chamber containing a particulate bed, of inert or reactive, finely divided, pulverulent solid material. Gas, forced upwardly through the diffusing medium into the fluidizing chamber under a sufficient predetermined pressure, fluidizes the particulates. The gas pressure required to accomplish this is determined, in part, by the nature and degree of fineness of the particulates to be fluidized. Other influencing factors are the depth of the bed and the size, number and design of the plenum chamber compartments and passages into the superimposed fluidizing chamber.
In conventional, static fluidized beds, by and large, the rate at which the fluidizing air can be blown through the bed is limited by the fact that only the gravity force on the bed particles opposes the balancing air force needed to maintain fluidization. In rotating fluidized beds, the fluidizing air forced through the bed opposes the centrifugal force tending to throw the bed particles outwardly from the bed axis of rotation toward the bed periphery. The extent of the centrifugal force and, thus, of the opposing fluidizing air flow rate can be controlled by controlling the speed of bed rotation. A form of rotating fluidized bed combustor system is disclosed by J. Swithenbank in his article "Rotating Fluidized Bed/Combustor/Gasifier". The Swithenbank system includes a vertical shaft around which rotates a generally cylindrical combustor using natural gas as the fuel. The gas is introduced at the center of the combustor, i.e., along the axis of rotation, and is mixed with fluidizing air forced through the bed particles from the bed periphery toward the center. The bed, which is heated by the combustion heat generated and the mixing action accompanying rotation, preheats the entering fluidizing air. Most of the combustion between the heated air and the natural gas appears to occur outside, rather than within, the bed itself. Cooling coils passing through the bed carry air which is heated by the combustion and serve to control the bed and exhaust gas temperature. Swithenbank states that his combustion system may be operated by burning or gasifying coal granules in the fluidized bed, but discloses no combustor configuration suitable for use with coal fuels. Moreover, Swithenbank's configuration, requiring introduction of the fuel along the axis of rotation, detracts from the attainment of maximum energy density because it diminishes the compactness of the system. See also, Demircan et al, Rotating Fluidized Bed Combustor, published in "Fluidization" by Cambridge University Press (1978). Other publications of interest in connection with the heat transfer and combustion characteristics of natural gas fueled rotating fluidized beds are J. Broughton and G. E. Elliott, Heat Transfer and Combustion in Centrifugal Fluidized Bed, I. Chem E. Symposium Series No. 43 (paper presented at June, 1975 meeting), and G. C. Lindauer et al, Experimental Studies on High Gravity Rotating Fluidized Bed, U.S. Atomic Energy Commission, BNL-50013 (Sept. 1966).
One of the important characteristics of a fluidized bed is its very high heat transfer. The values for the heat transfer coefficient between the bed and surfaces within the bed have been reported as high as 180 BTU/hr-ft.sup.2 -.degree.F. for a static fluidized bed. In rotating beds, heat transfer coefficients along surfaces within the bed may be on the order of 240 BTU/hr-ft.sup.2 -.degree.F. By comparison the heat transfer coefficient between a tube and gas flowing therewithin in conventional, non-fluidized bed environments is on the order of 10-25 BTU/hr-ft.sup.2 -.degree.F. Investigations have determined that the characteristics of rotating fluidized beds which lead to such unusually high heat transfer coefficients within the bed can be used to great advantage in a heat exchanger. In particular, the high heat transfer coefficient of surfaces in a fluidized bed suggests that there is potential for the application of rotating fluidized bed heat exchangers as alternatives to primary surface heat exchangers in gas turbine engines.
The foregoing illustrates limitations of the known prior art. Thus, it is apparent that it would be advantageous to provide an alternative to the prior art.